A closer look at scientific molding theory

Plastic injection processing has experienced massive change in the last 20 years. The days of molding by time and pressure have given way to molding by position, peak pressure and process repeatability. RJG and John Bozzelli were major pioneers in the development of decoupled and scientific molding. As the procedures were developed and given structure. Plastic injection as an "art" gave way to plastics processing as a science of repeatability and standardization.

In recent years, the term "scientific molding" has become a buzzword of sorts. There are many organizations claiming to be "scientific" molders and trainers. It is important to note here there are many variances from one training program to another. Great care should be taken when choosing a training source for scientific molding theory and applications. 

First, we need to understand what scientific molding is as an application. It is the science of process development, recording, standardization and repeatability. These variables are strictly dependent upon machine and mold validation and design. Poorly functioning machines, or molds that fail from a design perspective may inhibit or remove a manufacturers ability to develop a process that is repeatable. Moldflow is an excellent source of testing mold design prior to its inception and construction. From a machine validation standpoint, John Bozzelli and RJG have tools available for machine testing.

Once machine and tooling has passed validation, a process is developed using decoupled molding technique, which then is further developed as a science-based process and application. One analogy that describes the development of scientific structure is this:

We have all seen a pit full of balls that children love to jump into and cover themselves in. Imagine that one of these pits are filled with nearly all red balls (the red balls representing processes that do not work and produce scrap or cannot be controlled), and a few blue balls (The blue representing processes that are repeatable and profitable..efficient and little to no scrap). Imagine reaching into the pit with your eyes closed.. trying to grab a blue ball. With so many red balls, it takes time to finally grasp a blue ball in your hand. When you finally achieve this.. you take great pains to record everything that you had to do to grab the blue ball.. so that the next time you try to get it, it becomes easier and faster to obtain. In a nutshell, a science-based process is a process that has been validated as profitable and repeatable.. and following validation a "snapshot" of sorts is taken (through process variable recording) of all monitoring actuals to assure that the next time a press is set up, the process can be repeated via prior proven success and profitability. In essence, the more care we put into documenting every recordable condition from a previously successful run.. the better we are able to repeat that successfully.

The following variables should be recorded and monitored as scientific molding data pertaining to the plastic injection industry standards. When changes in molding conditions are noted, these variables can help determine the cause of change, and repeating previous molding variables often will return process to good running state. This article outlines basic scientific molding approach, and we will provide you with multiple reference sources to further examine the applications and theories of scientific and decoupled molding. Here are some of the primary variables that affect process consistency: 

 

Molding Variable

Description

Fill Time

Fill Time is the amount of time taken from the beginning of shooting material to the point of reaching cut-off

Peak Pressure

Peak Pressure is the maximum pressure achieved at the point of velocity cut-off prior to dropping off into hold pressure

Running Mold Temperature

Mold Temperature should be measured at various points in the mold in a running condition

Screw Rotate Time

Screw rotate time is the amount of time it takes for the screw to recover

Melt Temperature

See article: "Performing Melt Temperature study"

Cycle time

The amount of time taken for each shot to be produced

Cushion

Cushion should hold steady between .15 and .35 depending on part size

Water Pressure(To Process)

Gallons per minute measurement taken prior to mold entry

Water Pressure(From Process)

Compared to "to process" pressure for calculation of pressure drop

Barrel Temp Actuals

Actual running temperatures of barrel zones

Mold Open Time

Actual time mold is open between shots

Back Pressure (actual)

actual pressure held during recovery stage (PSI)

Material moisture

Material moisture in normal running condition

Regrind Percentage

Very important to maintain consistent regrind usage

Key Injection Molding Fundamentals:

The key to any successful molding operation is recording all data that is available when the process is producing minimal scrap and is at optimum efficiency. By replicating these variables at machine start-up, you assure that you are repeating your previous run. Here are some other factors that can help to determine the success of your operation:

It is important to note that there are several steps that are part of establishing a robust process. These studies include:

(a)Viscosity Curve
(b)Cavity Balance
(c)Pressure Drop Study
(d)Gate Seal
(e)Cool Time

There are instructions for performing several of these studies on the site and are also offered through Routsis, RJG and Bozzelli program offerings.

1. Melt temperature is a key variable that is often overlooked! It is imperative that once processes are validated, melt temp should be recorded and when problems occur, melt temp should be one of the primary checks made prior to process adjustments. It is one of the focuses that can identify problems with your machine, or changes within the molding environment.

2. Measure and record the GPM of your tool's circuitry...by measuring this variable on every circuit, you are able to test your mold in the future when molding conditions show signs of overheating. It can help you to determine if the tool requires descaling. Each circuit should have a unique identification number. This is done by repeating your watering procedure. Keep the same supply and return pattern by either hard plumming your loops, or by establishing a watering diagram that maps out the watering layout. It is also recommended that your circuits should be uniquely identified IN or OUT and use color identification (green,yellow,white,etc.). This not only improves your watering time, but will reduce the potential of miswatering.

3. A validated process should not require change. Process parameters should be able to be repeated each time a mold is set and started. Before changing your process, it is important to look at your monitoring variables first! What changed? For instance...if fill time is slower, look at your temperature actuals. Check your thermolator...is the actual temperature the same as your setpoint? Yes, there will be times you need to change your process to correct a molding condition. First, check for mechanical changes..after confirming that your machine, mold and auxilliary equipment are in a correct state, make changes to your process that replicate your process monitoring variables (fill, peak, etc.).

4. Prior to process changes, monitor your molding variables for significant fluctuations. A cushion that varies spurratically can be a sign of a worn check ring, or barrel wear. Barrel temperature fluctuations can point towards bad heater bands, or thermocouple positioning/ failure. Also consider recent mechanical changes made while servicing a press, and whether they might impact the validity of your process.

5. Always consider whether your operator could be affecting your process. Inconsistent cycles can reap havoc on your consistencies. Improper part handling can cause defects that might be mistaken for a processing problem.

6. Part weight is a key recordable variable. Once a process has been validated, part weight (full shot, including runner) should be recorded. The data should not only include part weight after cycle, but also part weight with pack and hold removed. This can help you identify where in the process you are experiencing a change. Part weight should be verified at the beginning of each start up.

7. A clean and well serviced mold is imperative to any successful molding operation. Tools should be cleaned no less than once per shift, and materials that are prone to gassing may require twice per shift. Slides and guide pins should be lubed, but it is important to remember that overgreasing can be detrimental to your process efficiency. Always clean your mold prior to any process change...defects could be directly related to dirty vents.

8. Material moisture is a key function that is often overlooked when process defects occur.Moisture analysis should be part of your start up procedure, and is completed prior to start up. Upkeep of your dryers is essential to your success. Dryer filters should be cleaned every shift, and you should routinely feel the supply and return hoses on your dryer. When a dryer is functioning properly, the supply hose will be hot and the return will be warm.

9. It is imperative that you analyze the effectiveness of your fill time. Some materials require a fast fill..but if you max out your velocities you lose control of consistency. Monitor your fill and verify that your setpoint is being reached consistently. Whenever possible, your fill speed should be determined by performing a fill time study. There is a form on the homepage of the site for this that includes instructions on performing the test.

10. Hold time is a crucial element of your cycle...establishing this time too low results in part weight variations and process inconsistency. Too much time adds time to your cycle that isn't needed. Performing a gate seal study not only verifies that you are achieving gate seal consistency, but is a crucial step towards process optimization.There is a form on the homepage of the site for this that includes instructions on performing the test.

11. It is important to understand that adding regrind to your process changes material response. The best approach to introducing virgin/regrind into your molding equation is to treat it as a different material. Determine the optimal virgin to regrind ratio that reutilizes your regrind effectively without increasing your scrap rate. Once you've established an effective blend and process, record the process separately from a virgin run. Also record the process monitoring data separately, then consistently repeat the blend ratio. You can further your ability to mold consistently by re-extruding your regrind with virgin base. This will reduce the potential of drop-down inefficencies (pellet size/ weight vs. regrind size/weight) and promote consistency.

Scientific molding is best described as the science of replication. Optimized and validated processes are repeatable...successfully producing low scrap start-ups and product runs, optimized cycle efficiencies and productive and profitable returns.

About the author: Garrett MacKenzie is the owner and editor of www.plastic411.com <http://www.plastic411.com> . Mackenzie started in plastics at the age of 19 as an operator, eventually moving up through the ranks to engineering over a 27-year timeframe. He currently is working as a plastic injection consultant in engineering capacities.

Comments (0)

Please log in or register to post comments.
  • Oldest First
  • Newest First
Loading Comments...